Before we go any further, in RNA there is NO thymine nucleotide (T in the genetic code) as there is in DNA.
Instead, there is a similar base called Uracil (U). So, in RNA, U replaces T and thus U pairs with A. You can
be sure, if you see a sequence of bases mentioned and a U is present (such as AUCGAAUU) that it is RNA
and NOT DNA that is being discussed, uracil is unique to RNA.
What is a Gene?
How is it possible that a singlecelled zygote can contain the information necessary to form the complete
individual? The answer lies in the nature of the DNA molecule. This unique chemical has a structure that
allows it to replicate prior to each cell division so that the daughter cells have the same chromosomes (and
genes) as the original zygote had. The structure of DNA also allows it to store information. The language
used to record the information has an alphabet of only 4 letters. These are the bases, adenine, guanine,
thymine and cytosine. Sequences of these 4 bases along the length of the chromosomes form the genes.
But what is the significance of a gene?
In the 1940s, many years before Watson and Crick described the structure of the DNA molecule, two
scientists named George Beadle and Edward Tatum received the Nobel Prize for developing the first clear
definition of a gene. They said one gene, one enzyme. In other words, a gene is a section of a DNA
molecule that codes for an enzyme in one of the chemical processes of a cell.
This is a very profound statement, and although the definition of a gene has been expanded and refined
over the years, the basic principle remains constant. Remember that enzymes are catalysts at each stage
of biochemical pathways. They bring the reactants together in the right way so that the reactions occur
quickly. It is believed that, given the correct enzymes in the correct order, and the raw materials or
substrates, all of the chemical reactions of the cell occur, and therefore all of the characteristics of the
organism are possible.
What we actually inherit from our parents is the information to make enzymes plus the other proteins of the
cell. Your text refers to one gene, one protein to encompass proteins other than enzymes, like hormones,
membrane proteins etc., in the definition of a gene.
How Proteins are Made (a simplified overview)
You could consider the DNA to be like a cookbook. It contains the recipes for all of the proteins that the cell
can possibly make over its lifetime. It also contains instructions for making the molecules that carry out
protein synthesis. (You could think of these molecules as the machinery necessary for the process.) DNA
also contains vast amounts of information that is not yet understood. If the DNA is the cookbook, then a messenger RNA or (mRNA) molecule is a copy of the recipe for one
protein. It is a strip of RNA complementary to a fairly short section of one side of a chromosome. mRNA is
made by transcription and then it leaves the nucleus. Once in the cytoplasm it may be used over and over
again to make many molecules of the same protein.
In general, each set of three bases on the mRNA strand codes for one amino acid. These sets of three are
called codons. Since the mRNA must be read in the correct direction (beginning to end and not backwards,
which would give a different amino acid sequence) one codon, AUG, always occurs near one end of the
mRNA and indicates the beginning of the recipe.
The machinery for making proteins consists of short strands of transfer RNA or tRNA, and organelles called
ribosomes, which are made of ribosomal RNA or rRNA and ribosomal proteins. The tRNA and the rRNA
are both made by transcription of genes in the nucleus. (Some genes, therefore, code for RNA rather than
protein.) The rRNA is specifically made in the nucleolus where it is then combined with proteins in the
cytoplasm to form ribosomes.Once a tRNA has bound with its specific amino acid it is referred to a
Much of the time, the tRNAs float around freely in the cytoplasm. Each different tRNA has a particular set of
3 bases called an anticodon, and a tRNA with a certain anticodon will only transport one specific kind of
amino acid out of the 20. When the tRNA encounters its correct amino acid in the cytoplasm, it attaches to
it by a fairly complex, ATPrequiring reaction.
The process of making a protein using the sequence of bases (recipe) on mRNA is called translation. One
half of the ribosome and then the other half attach to the mRNA at the appropriate end. Simply put, the
ribosome then moves along the mRNA and matches a complementary tRNA anticodon to each codon. In
doing so, the tRNA places the correct amino acid in sequence for the new protein. The ribosome provides
the enzymes and energy necessary to make the peptide bonds between the amino acids. Once a tRNA has
delivered its amino acid, it leaves the ribosome and picks up another molecule of its particular amino acid
for delivery to another ribosome.
(a) If a specific codon in an mRNA is GUU, then only a tRNA with the anticodon CAA can place its amino
acid in the growing polypeptide chain at that point. This tRNA only carries the amino acid valine. (I know
this by looking up the mRNA codon GUU in Fig 7.6.)
(b) Suppose a section of an mRNA has the following bases.
UUGGCAAAAUCA Separate this into codons. UUG GCA AAA UCA
Which amino acids does this mRNA code for?
(See Fig 7.6 to work out the amino acid sequence.)
leucine C alanine C lysine C serine
What is the anticodon on the tRNA that transports alanine? it is CG_
In summary then, the sequence of codons on the mRNA dictates the sequence of amino acids in the
polypeptide (protein). The role of the tRNA is to transport a specific amino acid into the ribosome at the
correct point. As the ribosome moves along the mRNA more and more amino acids are added to the
polypeptide chain. When it arrives at the stop codon, it releases the polypeptide and the two halves of the
ribosome separate from the mRNA. All of the machinery (mRNA, ribosome and tRNA) can be used over
In this chapter there is a description of the Barr body. Dr. Murray Barr was a professor at the University of
Western Ontario. He and his graduate student Dr. E.G. Bertram made the discovery of the Barr Body here
at Western and published their discovery in 1966.
Darwins thoughtful melding of his observations of the natural world with the ideas of other thinkers led him
to propose that evolution could occur by way of a pr